Synthesis of an analogue of the C-cluster of C. hydrogenoformans carbon monoxide dehydrogenase requires formation of a planar Ni-II site and attachment of an exo iron atom in the core unit NiFe4S5. The first objective has been achieved by two reactions: (i) displacement of Ph3P or (BuNC)-N-t at tetrahedral Ni-II sites of cubane-type [NiFe3S4](+) clusters with chelating diphosphines, and (ii) metal atom incorporation into a cuboidal [Fe3S4](0) cluster with a M-0 reactant in the presence of bis(1,2-dimethylphosphino)ethane (dmpe). The isolated product clusters [(dmpe)MFe3S4(LS3)](2-) (M = Ni-II (9), Pd-II (12), Pt-II (13); LS3 = 1,3,5-tris((4,6-dimethyl-3-mercaptophenyl)thio)-2,4,6-tris(p-tolylthio)b enzene(3-)) contain the cores [MFe3(mu(2)-S*)(mu(3)-S)(3)](+) having planar (MP2S2)-P-II sites and variable nonbonding M center dot center dot center dot S* distances of 2.6-3.4 angstrom. Reaction (i) involves a tetrahedral -> planar Ni-II structural change between isomeric cubane and cubanoid [NiFe3S4](+) cores. Based on the magnetic properties of 12 and earlier considerations, the S = 5/2 ground state of the cubanoid cluster arises from the [Fe3S4](-) fragment, whereas the S = 3/2 ground state of the cubane cluster is a consequence of antiferromagnetic coupling between the spins of Ni2+ (S = 1) and [Fe3S4](-). Other substitution reactions of [NiFe3S4](+) clusters and 1:3 site-differentiated [Fe4S4](2+) clusters are described, as are the structures of 12, 13, [(Me3P)NiFe3S4(LS3)](2-), and [Fe4S4(LS3)L'](2-) (L' = Me2NC2H4S-, Ph2P(O)C2H4S-). This work significantly expands our initial report of cluster 9 (Panda et al. J. Am. Chem. Soc. 2004, 126, 6448-6459) and further demonstrates that a planar M-II site can be stabilized within a cubanoid [NiFe3S4](+) core.